1. Field of the Invention
This invention relates to power amplifiers and more particularly to high-power, monolithic, integrated circuit (IC) power amplifiers.
2. Background Art
Certain telecommunications standards have been developed in the United States to provide high-speed digital access between customers and a central office. One example is the Asymmetric Digital Subscriber Loop (ADSL) standard, which provides a data rate of approximately 6 Mb in the direction from the central telephone office to the customer. As a result of the high data rate, ADSL has become one of the preferred standards for supplying, for example, Internet service over a standard copper wire twisted pair.
Although it offers a high data rate, the ADSL standard requires that the line-driving amplifier at the central office be able to supply a signal of approximately 20 dBm (power with respect to a reference level of 1 mW) in power with a peak-to-rms average voltage ratio (PAR) of 5.33:1. In order to accommodate the required power output level and PAR, two line-driving amplifiers, or a line-driver pair, are commonly used in a bridge configuration with a 1:2 step-up transformer between the amplifiers and the line.
One disadvantage of this conventional arrangement is its need for power as the number of customers grows. In the near future, for example, it is expected that several thousand customers may require ADSL service from the same central office. The possibility of expending several kilowatts of power just to operate the ADSL lined rivers has therefore become a major concern regarding this type of digital service. Much research effort has therefore been applied to the problem of increasing the efficiency of monolithic, integrated circuit (IC) power amplifiers for ADSL applications.
In one known circuit arrangement, the output stage of an operational amplifier is connected to an approximately constant supply voltage, whose supply voltage is greater in magnitude than the maximum zero-to-peak output voltage. The amount by which the supply voltage exceeds the maximum output voltage is commonly known as the voltage headroom required for the stage. For example, a typical value for the voltage headroom in prior art circuits is 3V. The zero-to-peak voltage for an ADSL amplifier in the central office is approximately 8.42V. The minimum power supply voltage for such an arrangement would therefore be approximately 11.42V. The supply voltage, given some initial voltage setting inaccuracy, may therefore be nominally 12V.
Because the average current required from each power supply (+/-12V) in the amplifier pair is approximately 28.5 mA for the ADSL central office, the minimum power possible for a 12V supply would be 4.times.12V.times.28.5 mA=1.36 W. This would be the minimum power given ideal "Class C" operation of the output stage and without providing additional power for any amplifier current bias circuits. Because ADSL power amplifiers require low output distortion--typically better than 70 dB for signal to noise-and-distortion ratios--"Class C" operation is not practical. It is therefore understandable why, despite considerable efforts, it has not been possible to reduce power consumption for ADSL central office line driving amplifier pairs significantly below 1.5 W. Similar problems of course arise, or may arise, even in systems that use standards other than the ADSL for enabling data exchange between some central system and a number of customers or other client systems large enough that it becomes advantageous to reduce the need for supply power to the various amplifiers.
What is needed is therefore an arrangement that would make possible a reduction in the power level for the amplifier arrangement as much as possible, and preferably to below 1.0 W, especially for systems following the ADSL standard. This invention provides such an arrangement.